Ultrasonic acoustic radiation force excitation for ultrasonic material property measurement and imaging

ABSTRACT

An ultrasonic diagnostic imaging system for shear wave measurement transmits push pulses in the form of a sheet of energy. The sheet of energy produces a shear wavefront which is a plane wave, which does not suffer from the 1/R radial dissipation of push pulse force as does a conventional push pulse generated along a single push pulse vector. The sheet of energy can be planar, curved, or in some other two or three dimensional shape. A curved sheet of energy can produce a shear wave source which focuses into a thin line, which increases the resolution and sensitivity of the measuring techniques used to detect the shear wave effect.

This invention relates to medical diagnostic ultrasound systems and, inparticular, to ultrasound systems which perform measurements of tissuestiffness or elasticity using shear waves.

Various means of remotely interrogating tissue mechanical properties fordiagnostic purposes have been developed that exploit the radiation forceof an ultrasonic beam to apply force remotely to a region of tissuewithin the body of the patient (acoustic radiation force; also referredto as “push” pulses). Acoustic radiation force can be applied in such away that elastic properties may be measured, either locally at the pointof deformation by tracking the deformation directly through the use offurther ultrasonic imaging to follow the pattern of deformationquasi-statically and visually discern regions of varying stiffness. See,for example, Nightingale, K. R. et al, “On the feasibility of remotepalpation using acoustic radiation force”, J. Acoust. Soc. Am., vol. 110no. 1 (2001), pp. 625-34; and M. L Palmieri et al. The deformationcaused by the acoustic radiation force can also be used as a source ofshear waves propagating laterally away from the deformed region, whichmay then be imaged to interrogate adjacent regions for their materialproperties through time-domain shear wave velocity imaging. See, in thisregard, Sarvazyan, A. et al., “Shear wave elasticity imaging: A newultrasonic technology of medical diagnostics”, Ultrasound Med. Biol. 24,pp 1419-1435 (1998) and “Quantifying Hepatic Shear Modulus In Vivo UsingAcoustic Radiation Force”, Ultrasound in Med. Biol., vol. 34, 2008. Thistechnique can also be used to assess frequency-domain shear wave modulusand viscosity. See Fatemi, M. et al., “Ultrasound-stimulatedvibro-acoustic spectrography”, Science 280, pp 82-85 (1998). Thesetechniques use a one dimensional array transducer to generate the shearwaves and are thus hampered by limited effective depth of penetration bya combination of weak coupling and safety limits imposed on the maximumpower of the excitation beams, combined with unfavorable diffractioneffects that limit the penetration depth for effective measurement. SeeBouchard, R. et al., “Image Quality, Tissue Heating, and Frame RateTrade-offs in Acoustic Radiation Force Impulse Imaging”, IEEE Trans.UFFC 56, pp 63-76 (2009).

Additionally, the existing techniques, due to the limited extent of thepush pulse excitation and the two dimensional imaging methodology, areincapable of discriminating between regions of property variationswithin the plane of imaging and those which may lie near but out of theplane. The mixing of these out-of-plane property values with in-planevalues during the imaging process may lead to unnecessary reduction ofaccuracy and diagnostic value in the output of these techniques.

In conventional acoustic radiation force imaging and pointquantification as presently practiced, the push is generated by a 1-Darray that produces a beam which may be well controlled in the singleimaging plane, but is restricted to a single, moderately tight focus inthe cross, or elevation plane, by a fixed focus mechanical lens. Thisleads to a mechanical push force which creates a response laterally inall directions, in and out of the plane of the array. The tissue motionelicited by this push propagates generally radially in all lateraldirections, and suffers a fall off as a factor of 1/R in the radialdirections (in the instance of a line source in the push pulsedirection) in addition to normal attenuation caused by tissue viscosity.In the case of acoustic radiation force qualitative and quantitativeimaging, this is deleterious because out-of-plane regions of stiffnessvariation will contribute to the axial displacement in the image plane,confounding the accuracy of the stiffness measurement in the imageplane. In the case of point quantification, the radial propagationdisburses useful shear wave energy away from the imaging plane, reducingthe signal amplitude needed for accurate property estimation.

The motion produced by acoustic radiation force transmission withindiagnostic emission limits is very small, on the order of 0.1 to 15micrometers in amplitude. The measurement of such tiny motions isaccomplished by tracking the reflections from local inhomogeneities inthe tissue being studied, which means that the received signal effectsof a shear wave can be difficult to discriminate. In addition, shearwave motion is heavily damped in tissue, which is viscoelastic incharacter. Thus, an adequate signal-to-noise ratio is difficult toobtain, and penetration range is very limited. Any interfering signalswill adversely affect the results. A significant source of interferenceis relative motion of the transducer being used for the study and theregion of tissue being studied. This can be caused by external sourcessuch as unsteadiness of the operator's hand, or internal sources such asbreathing, heartbeats, or other voluntary or involuntary movement of thesubject. Prior art attempts at signal-to-noise improvement for acousticradiation force techniques would bandpass filter the signals toeliminate the lower frequencies from the data. Most of the motionartifacts are below 50 Hz, so some improvement can be made. See, forexample, Urban et al, “Error in Estimates of Tissue Material Propertiesfrom Shear Wave Dispersion Ultrasound Vibrometry,” IEEE Trans. UFFC,vol. 56, No. 4, (April 2009). However, some of this interference isquite large in amplitude, and bandpass filtering is not alwayssufficient to eliminate the adverse effects. Artifacts in the form ofmis-estimated displacements and hence miscalculated shear wavevelocities and moduli are common.

Accordingly, it is an object of the present invention to improve theeffective depth of penetration of acoustic radiation force effects suchas shear waves. It is a further object of the present invention toreduce out-of-plane effects during material assessment. It is a furtherobject of the present invention to reduce measurement errors due to therelative motion of the transducer in acoustic radiation force-basedstudies.

In accordance with the principles of the present invention, a diagnosticultrasonic imaging system and method are described which enables a userto acquire highly resolved image data sufficient to measure tissuemotion or the characteristics of a shear wave propagating throughtissue. An ultrasound probe with a two dimensional array of transducerelements transmits a push pulse in the form of a sheet of energy intotissue. The sheet of energy can be planar or non-planar, and can beproduced by a sequence of individually transmitted ultrasound pulses orby transmission of a plane wavefront. Unlike the single vector pushpulses of the prior art, the two dimensional push pulse of the sheet ofenergy produces a planar or semi-planar shear wavefront which does notsuffer from the 1/R falloff of energy spread of the prior arttechniques. In accordance with a further aspect of the presentinvention, a plurality of background tracking pulses are transmittedabout the location of a push pulse and the field of interest in which ashear wave is to be detected. Echo signals received from the backgroundtracking pulses are correlated over time to estimate background motionin the field of interest during propagation of the shear wave, which isused to adjust the measured displacement caused by passage of the shearwave.

In the drawings:

FIG. 1 illustrates in block diagram form an ultrasonic diagnosticimaging system constructed in accordance with the principles of thepresent invention.

FIGS. 2a-2c illustrate the transmission of a sequence of push pulses todifferent depths to produce a shear wavefront.

FIG. 3 spatially illustrates a sequence of pulse pulses along a pushpulse vector, the resultant shear wavefront, and a series of trackingpulse vectors.

FIG. 4 illustrates the radial spread of a shear wavefront emanating froma push pulse vector.

FIG. 5 illustrates a two dimensional push pulse produced in accordancewith the principles of the present invention.

FIG. 6 illustrates a curved two dimensional push pulse produced inaccordance with the principles of the present invention.

FIGS. 7-9 illustrate the use of background tracking pulses to estimatebackground tissue motion in the region of a shear wave in accordancewith the principles of the present invention.

Referring first to FIG. 1, an ultrasound system constructed inaccordance with the principles of the present invention for themeasurement of shear waves is shown in block diagram form. An ultrasoundprobe 10 has a two dimensional array 12 of transducer elements fortransmitting and receiving ultrasound signals. A two dimensional arraytransducer can scan a two dimensional (2D) plane by transmitting beamsand receiving returning echo signals over a single plane in the body andcan also be used to scan a volumetric region by transmitting beams indifferent directions and/or planes of a volumetric (3D) region of thebody. The array elements are coupled to a micro-beamformer 38 located inthe probe which controls transmission by the elements and processes theecho signals received from groups or sub-arrays of elements intopartially beamformed signals. The partially beamformed signals arecoupled from the probe to a multiline receive beamformer 20 in theultrasound system by a transmit/receive (T/R) switch 14. Coordination oftransmission and reception by the beamformers is controlled by abeamformer controller 16 coupled to the multiline receive beamformer andto a transmit controller 18, which provides control signals to themicro-beamformer. The beamformer controller is responsive to signalsproduced in response to user manipulation of a user control panel 40 tocontrol the operation of the ultrasound system and its probe.

The multiline receive beamformer 20 produces multiple, spatiallydistinct receive lines (A-lines) of echo signals during a singletransmit-receive interval. The echo signals are processed by filtering,noise reduction, and the like by a signal processor 22, then stored inan A-line memory 24. Temporally distinct A-line samples relating to thesame spatial vector location are associated with each other in anensemble of echoes relating to a common point in the image field. Ther.f. echo signals of successive A-line sampling of the same spatialvector are cross-correlated by an A-line r.f. cross-correlator 26 toproduce a sequence of samples of tissue displacement for each samplingpoint on the vector. Alternatively, the A-lines of a spatial vector canbe Doppler processed to detect shear wave motion along the vector, orother phase-sensitive techniques such as speckle tracking in the timedomain can be employed. A wavefront peak detector 28 is responsive todetection of the shear wave displacement along the A-line vector todetect the peak of the shear wave displacement at each sampling point onthe A-line. In a preferred embodiment this is done by curve-fitting,although cross-correlation and other interpolative techniques can alsobe employed if desired. The time at which the peak of the shear wavedisplacement occurs is noted in relation to the times of the same eventat other A-line locations, all to a common time reference, and thisinformation is coupled to a wavefront velocity detector 30 whichdifferentially calculates the shear wave velocity from the peakdisplacement times on adjacent A-lines. This velocity information iscoupled into a velocity display map 32 which indicates the velocity ofthe shear wave at spatially different points in a 2D or 3D image field.The velocity display map is coupled to an image processor 34 whichprocesses the velocity map, preferably overlaying the anatomicalultrasound image of the tissue, for display on an image display 36.

FIGS. 2a-2c illustrate the transmission of a sequence of focused high MI(Mechanical Index) push pulses (e.g., MI of 1.9 or less so as to bewithin FDA diagnostic limits) along a single vector direction to producea shear wavefront. Pulses of high MI and long durations are used so thatsufficient energy is transmitted to displace the tissue downward alongthe transmit vector and cause the development of a shear wave. In FIG.2a the probe 10 at the skin surface 11 transmits a first push pulse 40into the tissue with a beam profile 41 a, 41 b to a given focal depthindicated by the shaded area 40. This push pulse will displace thetissue at the focus downward, resulting in a shear wavefront 42emanating outward from the displaced tissue.

FIG. 2b illustrates a second push pulse 50 transmitted by the probe 10along the same vector and focused at the deeper depth of the shaded area50. This second push pulse 50 displaces the tissue at the focal depth,causing a shear wavefront 52 to emanate outward from the displacedtissue. Thus, shear wavefronts 42 and 52 are both propagating laterallythrough the tissue, with the initial wavefront 42 preceding the secondas a function of the time interval between the transmission of the twopush pulses and the propagation delay difference due to the change inpropagation distance to the focus.

FIG. 2c illustrates the transmission by probe 10 of a third push pulse60 at a greater depth which produces an outward emanating shearwavefront 62. It is seen in FIG. 2c that the composite wavefront of thethree push pulses, indicated by the composite wavefront profile of 42,52, and 62, extends for an appreciable depth in the tissue, from theshallow depth of the first push pulse 40 to the deepest depth of thethird push pulse 60. This enables shear wave measurement over anappreciable depth of tissue. In an implementation of the system of FIG.1, a push pulse sequence such as this can be used to detect shear wavepropagation over a depth of 6 cm., a suitable depth for breast massimaging and diagnosis.

It will be appreciated that a greater or lesser number of push pulsescan be transmitted along the push pulse vector, including a single pushpulse. Multiple push pulses can be transmitted in any order, with theorder determining the shape and direction of the composite shearwavefront. For example, if the push pulses of FIGS. 2a-2c weretransmitted in sequence from the deepest (60) to the shallowest (40)with appropriate delays between transmits, the composite shear wavefrontof FIG. 2c would have the inverse tilt to that shown in FIG. 2c .Typically, each push pulse is a long pulse of 50 to 200 microseconds induration. A typical duration is 100 microseconds, for instance. Theultrasound produced during the 100 microsecond pulse duration arecompressional wave pulses and can have a frequency of 7 or 8 MHz, forexample. The push pulses are well focused, preferably at an f number of1 to 2. In one typical implementation, a push pulse is transmitted every2.5 milliseconds (as long as the shear source moving speed from (40) to(50) and (50) to (60) is greater than the shear wave propagation speed),giving the push pulses a 400 Hz transmission frequency. In anotherimplementation, all three push pulses are transmitted in one sequence tolaunch the full shear wavefront before the tracking A-lines begin.

FIG. 3 is another illustration of the use of three push pulses to createa composite shear wavefront. The three push pulses are transmitted alongvectors 44, 54, and 64 which are seen to be aligned along a singlevectorial direction in FIG. 3. When the deepest push pulse of vector 64is transmitted first followed by push pulses focused at successivelyshallower depths, the shear wavefronts of the respective push pulseswill have propagated as indicated by waves 46, 56, and 66 by a timeshortly after the last push pulse (vector 64) has been transmitted. Thetime intervals between the transmit times of the push pulses isdetermined by the shear and longitudinal velocities, because thepropagation time to the focus needs to be taken into account. As theshear waves 46, 56, and 66 travel outward from the push pulse vector,they are interrogated by tracking pulses 81 shown in spatial progressionalong the top of the drawing. Tracking pulses can occur between as wellas after push pulses. Unlike the depiction of FIG. 2c , the illustrationof the shear waves 46, 56, and 56 of the composite wavefront of FIG. 3shows the propagated shear waves to be substantially aligned in time andhorizontal propagation distance. From the perspective of the dramaticdifference in propagation speed between the longitudinal push pulses andshear waves in tissue, on the order of 100 to 1, this is arepresentative depiction when the individual push pulses are transmittedin rapid succession. Since the sole function of the push pulses is toeffect a force on tissue and no following time period is needed for echoreception as is the case with pulse-echo ultrasound imaging,substantially no dead time is required following each pulse and the pushpulses can be transmitted in very rapid succession. The transit time ofa push pulse in tissue is on the order of 100 microseconds (ultrasoundtravels at a speed of about 1560 meters/sec in tissue), whereas a shearwave period in tissue is on the order of 2 to 10 milliseconds (shearwaves travel at a speed of about 1-5 meters/sec in tissue). Thus, fromthe perspective of the periodicity and speed of a shear wave, a rapidsuccession of push pulses is near instantaneous, producing a singlewavefront.

In conventional acoustic radiation force imaging and pointquantification, the push pulse(s) are transmitted along a single vectordirection. When the push is generated by a 1-D array, a transducerhaving a single line of transducer elements, the array produces a beamwhich may be well controlled in the single imaging plane of the array,but is restricted to a single, moderately tight focus in the cross, orelevation plane by the fixed focus mechanical lens of the probe. Thisleads to a mechanical push which creates a response which radiateslaterally in all directions, in and out of the single imaging plane ofthe array. The tissue motion elicited by this push energy propagatesroughly radially in all lateral directions as illustrated by thecircular wavefronts 72 surrounding the push pulse vector and theoutward-radiating arrows 70 in FIG. 4, and suffers a fall-off in energyas a function of 1/R in the radial directions in addition to normaltissue attenuation. In the case of acoustic radiation force qualitativeand quantitative imaging, this is deleterious because out-of planeregions of stiffness variation will contribute to the tissue axialdisplacement in the image plane, confounding the accuracy of thestiffness measurement in the image plane. In the case of acousticradiation force point quantification, the radial propagation removesuseful shear wave energy from the imaging plane, reducing the signalamplitude needed for property estimation.

In accordance with the principles of the present invention, the pushpulse is formed as a two dimensional sheet of energy rather than asingle one dimensional vector. Such two dimensional push beam sheetsextend in the depth dimension D and also in the elevation or azimuthdimension E as illustrated by push beam sheet 80 in FIG. 5. The pushbeam sheet 80 results in the generation of shear waves with planarwavefronts as indicated by plane wavefronts 90, 92 in FIG. 5, whichtravel laterally from the force field of the push beam sheet 80 asindicated by arrows 91, 93. This shear wave excitation is like a planewave source rather than the line source of FIG. 4, eliminating the 1/Rfall-off in radial energy dissipation. The programmability andresponsiveness of the two dimensional array 12 to form beams inarbitrary directions and from apparent centers in various locations onthe surface of the array is used to generate pushed tissue regions ofgeneral shape, size, and direction of push by axial and/or lateralsweeping of the focal spot, rapid hopping from one focal spot to fromone location to another, or both, taking advantage of the dramatic ratioof propagation speed between longitudinal push waves and shear waves intissue (order of 100 to 1) to allow formation of an effective source ofshear waves which may be of somewhat arbitrary size, shape andorientation such that a focused and steered two- or three-dimensionalshear wave beam source of desired orientation, shape and extent may beformed.

In the simple implementation of the present invention shown in FIG. 5, aflat, extended sheet of pushed tissue is excited by push beam sheet 80,which generates shear waves in a flat sheet 90, 93, propagatinglaterally rather than radially outward, and decreasing the decay withtravel distance of the shear wave. This improves the distance ofpenetration of the various radiation force modalities. This sheet may beformed by focusing deeply within the tissue and starting thetransmission of a long burst of ultrasound. While the burst is beingtransmitted, the focal point is drawn shallower toward the transducer toform a line source. Multiple such lines of push beam force aretransmitted within a plane perpendicular to the face of the transduceras shown in FIG. 5. Alternatively the plane of push beam force istransmitted in other planes not perpendicular to the face of thetransducer array and within the range of directivity of the array, togenerate a planar source of shear waves. Such transmission willeffectively produce a unitary push force in two dimensions, so long asthe duration of the whole excitation sequence is somewhat faster thanthe period of the shear waves to be produced. Since longitudinalultrasound propagation path transit times are of the order of 100microsec, while the desired shear wave period is of the order of 2millisec, there is time for numerous transmits to produce the energysheet 90, 92.

A variation of the transmit technique of FIG. 5 is to transmit a sheetbeam which, by simultaneous excitation of elements of the twodimensional transducer array in elevation or azimuth, transmits a sheetbeam from the two dimensional array. Since the delay profile of the 2-Darray is fully programmable, transmitting a sheet focused deep in thefield and then moving the focal point closer at a rate comparable to theshear wave velocity will allow formation of a simple planar shear wavesource. This planar source may be transmitted at any rotational angle,so shear waves may be propagated in any lateral direction. Also, thetilt of the planar source may be varied, so the shear wave source may bedirected into planes not perpendicular to the array.

A third implementation of the present invention is illustrated in FIG.6. In this implementation a sheet beam is transmitted by the twodimensional array transducer which is transversely curved, either inspace or in delay profile or both, so that the resulting shear wavesource focuses into a thin beam, further increasing the resolution andsensitivity of the techniques used to detect it. It is even possible tocreate a curvature along the axial direction as shown by the push beamsheet (PBS) in FIG. 6, creating two dimensional focusing of the shearwaves. As this drawing illustrates, the two dimensional transducer array12 generates a curved push beam sheet PBS. The curvature of PBS causesthe shear wave front SWF to progressively converge as it travels, asindicated by the progressive convergence of SWF1, SWF2, and SWF3 towardthe checkered plane 98. This convergence is also indicated by theprofile 96 of the curved shear wave fronts. To the right in the drawingis a shear wave front SWF2′, illustrating the reverse curvature of theshear wave front as it passes beyond its line of maximum convergence atSWF3. This method of focusing the shear waves is most suitable for aline measuring technique rather than a plane measuring technique. Thedata collection rate is sharply reduced in exchange for the largeincrease in sensitivity in the vicinity of the shear wave focus at SWF3.This method can also be used to focus a two dimensional curved shearwavefront to a diffraction-limited point focus or a limited axial depthregion.

The diagnoses of tissue stiffness which are performed by measuring shearwaves are highly dependent upon precise tracking of the shear wavefrontover time, so that its changes in propagation velocity as it passesthrough different tissues can be accurately measured. In systems of theprior art, these measurements were performed while assuming that therewas no relative motion between the ultrasound probe and the tissue, sothat the only relative tissue motion is that produced by the push pulseforce. This assumption is often incorrect, since relative motion canalso be produce by unsteady holding of the probe, patient motion, oranatomical motion due to breathing and heartbeat motion. Thedisplacement caused by radiation force is very small, on the order of 10μm. Although the precision of ultrasound RF tracking can reach 1-2 μm,the shear wave motion can be buried in much larger patient motion suchas cardiac and respiratory motion, as well as environmentalinterference. While filtering can be used to try to eliminate noisewhose frequency is outside the range of the shear wave harmonicfrequencies, in accordance with a further aspect of the presentinvention, an additional step is taken to reduce noise. This consists ofusing the displacement estimated away from the region of the excitation(for instance, at a depth of at least half of the depth-of-field awayfrom the focus in the depth direction) as the background noise, since itcan be assumed that no significant radiation force is applied in thatregion. This noise “source” in the form of displacement estimates issubtracted from the shear wave displacement estimated at the region ofinterest.

A simple example of background motion sensing is illustrated in FIG. 7.A single vector push beam, being focused, has its most significanteffect along its beam axis near the focal depth 110. FIG. 7 illustratesthe profile 100 of a vector push beam in which the force of the pushbeam is concentrated. Some acoustic radiation force-based elastographictechniques involve only tracking along the same axis as the push beamand in this case, data from the tracking beams already in use can beemployed to sense background motion, but from ranges significantlyshorter and longer than the focal length, that is, outside the depth offield of the focused push beam, to make an axial motion estimate forsubtraction from the sample measurement. The stars 102 and 104illustrate focal regions of two background tracking beams, one locatedabove the focal region of the push pulse and one located below. Thefocal regions of the background tracking beams are indicated by thedashed beam profiles on either side of the background tracking beams.Echoes from these background tracking locations are sampled at aplurality of times before, during, and/or after transmission of the pushpulse(s). These temporally different echoes are compared, generally bycorrelation, and the comparison(s) used to assess the presence of axialbackground motion. Any displacement of the tissue due to backgroundeffects is subtracted from motion estimates due to the shear wave tocorrect estimated shear wave motion for background effects.

FIG. 8 illustrates another example of background motion sensing. In thisexample, additional locations 106, 107, 108 and 116, 117, 118 locatedlaterally well outside the region of interest 120 of shear wave trackingmay be tracked during the measurement interval to acquire data whichenables computation of a background motion estimate at any locationwithin or around the region of interest. Motion effects from tilting orrotation of the probe during the measurement interval can be sensed inthis way. For instance, if comparisons of echo changes at points 106,107, 108 over time indicate an upward motion to the left of the regionof interest 120, and differences at points 116, 117, 118 indicate adownward motion to the right of the region of interest at the same time,it can be concluded that there is an overall rotational or tiltingmotion of the probe with respect to the region of interest for whichcompensation in measurements should be made.

As FIG. 9 illustrates, it is also possible, and in some instances may bedesirable, to track in the plane of a 2D image along several lineswithin the region of interest 120 adjacent to the push region 100, attimes before and after the push event. In this example two lines ofbackground motion tracking samples 126, 127, 128 and 136, 137, 138 aresampled periodically during the measurement interval to the left of thepush pulse vector 100 and two lines of background motion trackingsamples 146, 147, 148 and 156, 157, 158 are sampled to the right of thepush pulse vector. Normally, one background motion sampling before andseveral background motion samplings after a push event are required toobtain a motion estimate due to a push event. However, if two or morebackground motion sampling ensembles before the push are acquired, anestimate of background motion can also be obtained. If at least onebackground motion echo ensemble is also acquired a sufficiently longtime after the push, additional estimates of background motion may bealso be gained, since motion may be interpolated in time from before toa time after the push pulse, rather than extrapolated. This techniquemay be performed at multiple lateral offsets from the axis of the pushbeam. If the background motion is not uniform over the region ofinterest, a scalar field estimate of the axial motion component withinthe sample volume may be obtained.

It will be appreciated that background motion correction can beperformed for measurements made in 3D space in addition to just a plane.The use of a two dimensional array transducer as shown in FIG. 1 enablesthe performance of elastography in three dimensions to bring improvedclinical utility, as out-of-plane variations in elasticity propertiescan adversely affect the effectiveness of single-plane elastographicmeasurements. The additional 3D control of push beam geometry by a twodimensional array can enhance signal-to-noise performance and bringadditional functionality. In this case, additional background motiontracking beams outside the region of interest during the measurementinterval and/or early and late background motion tracking beams may beadded within the 3D region of interest as indicated above for the 2Dcase, to obtain a full 3D volume estimate of axial motion for correctionof the measured response to the push beam excitation. For example, fourbackground motion tracking lines can be transmitted at 90° intervalsaround a push pulse vector. Background motion tracking lines can betransmitted in front of and behind a 2D push pulse sheet as describedabove to sense tissue motion in a three dimensional space over whichshear waves are being measured.

What is claimed is:
 1. An ultrasonic diagnostic imaging system for shearwave analysis comprising: an ultrasonic array probe having a twodimensional array of transducer elements which are operable to transmitultrasound waves along a plurality of different vectors in a subject;and a beamformer coupled to the two dimensional array to cause the arrayto transmit a burst of ultrasound waves and move a focal point of theburst during transmission of the burst to form each of a plurality ofline sources, wherein the beamformer is configured to simultaneouslyexcite the two dimensional array of transducer elements in twodimensions of the array to form a sheet of energy comprised of theplurality of line sources in a push pulse region for generating a shearwave that is one of: (1) a planar shear wave or (2) a semi-planar shearwave, and propagates away from the sheet of energy wherein thebeamformer is further configured to cause the two dimensional array oftransducer elements to transmit tracking pulses along tracking lineswithin a path of the shear wave and background tracking pulses alongbackground tracking lines outside the path of the shear wave withoutmoving the ultrasonic array probe, wherein the probe is furtherconfigured to receive echo signals from points along the tracking linesand background tracking lines, wherein the probe is further configuredto transmit the tracking pulses and receive the echo signals from pointsalong the tracking lines in a first time interleaved sequence, andwherein the probe is further configured to transmit the backgroundtracking pulses and receive the echo signals from points along thebackground tracking lines in a time interleaved sequence.
 2. Theultrasonic diagnostic imaging system of claim 1, wherein the sheet ofenergy is a planar sheet of energy.
 3. The ultrasonic diagnostic imagingsystem of claim 1, wherein the shear wave is the semi-planar shear wave,and wherein the semi-planar sheet of energy is a curved sheet of energy.4. The ultrasonic diagnostic imaging system of claim 1, wherein shearwave is the planar shear wave.
 5. The ultrasonic diagnostic imagingsystem of claim 4, wherein the sheet of energy is formed in a planewhich is normal to a face of the two dimensional array transducer. 6.The ultrasonic diagnostic imaging system of claim 4, wherein the sheetof energy is formed in a plane which is tilted at a non-orthogonal angleto a face of the two dimensional array transducer.
 7. The ultrasonicdiagnostic imaging system of claim 1, wherein the two dimensional arrayis configured to transmit the sheet of energy during a time intervalwhich is less than the time of one cycle of the generated shear wave. 8.The ultrasonic diagnostic imaging system of claim 7, wherein the focalpoint of the burst is moved both axially and laterally during the timeinterval.
 9. The ultrasonic diagnostic imaging system of claim 1, shearwave is the semi-planar shear wave, and wherein the semi-planar sheet ofenergy is a curved sheet of energy which produces a shear wave sourcefocused into a line.
 10. The ultrasonic diagnostic imaging system ofclaim 3, wherein the curved sheet of energy produces a shear wave sourcefocused to a diffraction-limited point focus.
 11. The ultrasonicdiagnostic imaging system of claim 3, wherein the curved sheet of energyproduces a shear wave source focused over a limited axial depth region.12. The ultrasonic diagnostic imaging system of claim 1, furthercomprising: an A-line memory for storing the tracking line echo data;and a display for displaying results of a shear wave measurement. 13.The ultrasonic diagnostic imaging system of claim 1, wherein the probeis configured to form the sheet of energy by sweeping the focal point inan axial direction, a lateral direction, or both.
 14. The ultrasonicdiagnostic imaging system of claim 1, wherein the probe is configured toform the sheet of energy such that the sheet of energy extends in adepth dimension as well as an elevation dimension.